I get an idea, which I either photograph or look up by typing key words into google images. If I can't find an image, I paint it from memory and work out some sketches. This usually occurs when I've glanced something I can't return to or photograph, or when I can't obtain a photograph of it, like something on the highway on a remote road-trip or an obscure slice of an obscure painting at the Met.
I then paint as closely as I can, the original idea. If it's from a photo, I stick slavishly to the photo, in a submissive way. If I have to come up with it from memory or a sketch, the process is much more fraught and angst-ridden, with no confidence that what I'm making is actually mine. I think it's because in the process, I'm grasping for straws and may allow another artist's work to grant me permission to do things. I hate that. Working from a photo takes my consciousness out of it.